Significant Accounting Policies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) item	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies
Minimum period for maturity dates for short-term investments	90 days
Accumulated deficit	$ (435,684)			$ (426,968)
Revenue recognition
Number of projects currently in production that could provide future variable consideration | item	0
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Significant Accounting Policies
Accumulated deficit			$ 0
Restatement adjustment | Accounting Standards Update 2016-02
Lease accounting
Operating lease liabilities		$ 185
Right of use operating assets		$ 185
Option Agreement Two | Guadalupe De Los Reyes, Mexico
Revenue recognition
Value of carrying value for option payment proceeds to be recognized as contract liability	$ 0